The present invention is directed to a bicycle brake device and, more particularly, to a bicycle brake device of the type that brakes a wheel by pressing first and second opposing brake shoes against the wheel in response to operation of a control cable having an inner cable that moves relative to an outer casing.
The shoe-pressure type of brake devices used in bicycles generally brake the wheels by pressing a pair of opposing brake shoes against the wheel when an inner cable positioned on the inside of an outer casing is pulled. With a brake device such as this, a pair of brake links is swingably attached to the frame or the like. Shoe attachment components are attached so that they each face one of these brake links, and brake shoes are mounted on these shoe attachment components. The distal end of an inner cable or the distal end of an outer casing is stopped at the brake links. When the brake lever attached to the handlebar is operated and the inner cable is pulled, the brake links swing in the direction in which they draw nearer to each other, and this swinging causes the brake shoes to press against the wheel. The amount of movement of the brake shoes, that is, the feed of the brake links (the amount they swing), increases roughly in proportion to the amount the inner cable is pulled.
A gap of several millimeters, for example, is required between the brake shoes and the lateral surfaces of the wheel rim. The purpose of this is to prevent the brake shoes from coming into contact with the rim as a result of axial runout of the rim or the like. Because of this gap between the brake shoes and the lateral surfaces of the wheel rim, there is a time lag between the operation of the brake lever and the point when the brake shoes actually hit the rim and braking commences. In order to minimize this time lag, one known brake device disclosed in U.S. Pat. No. 4,765,443 is equipped with a rapid traverse mechanism that quickly moves the brake links at the start of braking. This brake device is equipped with a pair of left and right brake links, a return spring, and a rapid traverse cam positioned between the brake links. Rollers that each have a cam groove formed in its peripheral surface are mounted to the upper ends of the brake links, and these cam grooves engage with the rapid traverse cam. Brake shoes are mounted to the lower ends of the brake links, and a swing center mounted on a seat provided to the bicycle fork is provided in the center of each brake link. A return spring is provided in order to bias the brake links to the brake release side. Adjustment of the spring force of this return spring allows the swing balance (feed balance) of the brake links in the release position to be adjusted and allows the braking commencement position of the brake links to be made even.
The rapid traverse cam is linked to the inner cable and rises when the inner cable is pulled. The rapid traverse cam has on both edges a cam component that engages with the cam groove in each of the brake links. The spacing of these cam components gradually widens from the top to the bottom, with a less rapid widening portion in between. Because the brake shoes are mounted below the swing center, the brake shoes are pressed against the wheel when the brake links swing in the opening direction. Accordingly, when the inner cable is pulled and the rapid traverse cam is moved, the brake links both open, with the amount of opening (feed amount) of the brake links at first being large with respect to the amount the inner cable is pulled, so that the brake shoes quickly strike the lateral surfaces of the rim. As the rate of widening of the cam components decreases, the rate of opening of the brake links with respect to the amount the inner cable is pulled also decreases, and there is a corresponding increase in the pressing force of the brake shoes on the rim.
With this type of structure, if an attempt is made to obtain a powerful braking force, the specified brake link feed amount will not be obtained unless there is an increase in the vertical length of the cam, at least in the portions of the cam where the rate of widening decreases. In actual practice, however, when the vertical length of the cam is thus increased, it can cause problems such as mud adhering, or the lower portion of the cam coming into contact with the wheel at the start of braking. Consequently, there is a dimensional limit to how much the braking force can be raised by lengthening the slower widening portion of the cam, and it is difficult to obtain a high braking force by means of this known cam structure.
Furthermore, since the rate of movement of the brake shoes varies for a given amount of upward movement of the cam, it is necessary that the cam be positioned correctly, i.e, perfectly symmetrical with the vertical axis passing through the wheel. If the cam is misaligned in any way (e.g., inclined relative to the vertical axis or laterally offset to one side of the vertical axis), then one brake shoe may move at a different rate than the other brake shoe, and that brake shoe may contact the rim before the other brake shoe. This, in turn, means that the desired braking force will not be obtained, and one of the brake shoes will wear out faster than the other.